The requirement for isolation of radio frequency (RF) energy from electronic circuitry has existed since the beginning of the electronic age. For every circuit that generates, and is sensitive to, this form of energy, there is a desire to electrically isolate this circuit from the external environment in which it operates. In general, it is highly desirable to minimize the effect that a given circuit has on its environment as well as the effect that the environment has on the circuit. In particular, when designing a shield for a frequency control device, such as an oscillator circuit, it is critical to isolate the circuit from other RF sources which may be emitting signals whose primary or harmonic frequencies are found near the frequency of oscillation.
The aforementioned requirement is especially true for components used in communication system's where even small levels of RF radiation may cause degradation in critical electrical parameters of the target circuit. This problem is accentuated when shielding must be provided while maintaining a hermetically sealed, or airtight, joint between the cover and the substrate holding the electronic circuit, such as in a quartz crystal resonator, or filter. Hermeticity is required in order to prevent air contaminants from coming in contact with the oscillating quartz plate, as even microscopic particulates have a dramatic impact on the frequency characteristics of the device. This requirement presents the added challenge of designing a highly manufacturable process (one with minimal material utilization) to provide an electrical path between the ground plane of the electrical circuit and the shielding enclosure. Though it represents a serious challenge to the electronic Package designer, such a ground path is a necessary feature if the component is expected to comply with the ever-increasing parametric specifications regarding attenuation of extraneous RF energy. An example of this is the noise-floor, or ultimate-attenuation, specification for a quartz crystal filter, which greatly depends on the effectiveness of the RF shielding provided. In particular, there are some applications which require a 65-70 dB noise floor, a level which is virtually unattainable without adequate shielding.
U.S. Pat. No. 4,739,453, "Shielding Apparatus for a Printed Circuit Board" suggests the use of plated through-holes, or vias, separated by a predetermined distance, to serve as an RF shield. These through-holes, whose spacing determines the wavelength of signals that are shielded, are used to provide an electrical path between the respective ground planes on several layers of a multiple layer printed circuit board. This technique addresses only systems using metallic cases for shielding, and does not address any systems having hermeticity requirements, such as the aforementioned frequency control devices.
Shielding for components such as quartz resonators and filters is typically provided by a conductive metal enclosure, or lid. The lid is then affixed to the substrate holding the circuit using processes which are well known in the art. Unfortunately, such metal lids are often cost prohibitive in the manufacturing of such components, where the cost of a stamped metal lid often exceeds 20% of the total product cost. In todays market, where competitive pricing is not a luxury, but a necessity for market survival, product cost reductions are often the manufacturers highest priority. Focusing on the cost problem, many manufacturers have begun using a molded lid, often constructed of a non-conductive material, such as ceramic. Such materials also perform better than metal through the reflow process, where undesired wicking of solder-based metallization cannot otherwise be adequately controlled. Unfortunately, the very nature of the non-conductive material further compounds the problem of providing adequate shielding, as electrical conduction, and subsequent dispersal of undesired RF energy, is the foundation of any good shielding system.
Accordingly, there exists a dire need for a cost effective, easily manufacturable, means for shielding an electronic circuit in a non-conductive enclosure from extraneous RF energy, while complying with any hermeticity requirements, such as those found in frequency control devices.